Fuel control



June 27, 1967 0'. F. WHEELER ETAL 3,327,756

FUEL CONTROL Filed July 1, 1965 2 Sheets-Sheet l INVENTORS 01%1/ EMHZ-ABA A T P EYS June 27, 1967 D. F. WHEELER ETAL 3,327,756

FUEL CONTROL Filed July 1, 1965 2 Sheets-Sheet 2 INVENTORS P54 Ph/f/EEAEE BY JOHN F. (HS 6W)? A TTORNE Y5 United States Patent 3,327,756FUEL CONTROL Dean F. Wheeler, Detroit, and John F. Vismara, GrossePointe, Mich., assignors to Holley Carburetor Company, Warren, Mich., acorporation of Michigan Filed July 1, 1965, Ser. No. 468,796 Claims.(Cl. 15811) This invention relates generally to turbine engine fuelcontrols, and more particularly to a novel fuel control for supplyinggaseous type fuels to stationary industrial turbine engines.

Since the use of gaseous fuel is generally more economical than usingliquid fuel, it has become the practice to adapt stationary power plantsfor operation on both gaseous and liquid fuel, the intention being thatthe engine will be operated on gaseous fuel as long as a suificientsupply thereof is available. Thus, when the use of natural gas forindustrial purposes must be reduced, as it often is during lateafternoon when household use thereof reaches its peak, the engine can betemporarily switched over to liquid fuel until such time as the naturalgas supply for industrial uses again becomes adequate. The switch toliquid fuel would, of course, be made at any time that the natural gassupply is inadequate, and the switch from gas to liquid, and vice versa,must be smooth and instantaneous.

Under these circumstances, there is a need for a fuel control adapted toproperly supply gaseous fuel such as natural gas, rather than liquidfuel, to a gas turbine engine in quantities suflicient to meet theenergy requirements of the engine over the entire range of operationthereof. To assure continuous operation of the power plant, it isnecessary that the gaseous fuel control may be operated in conjunctionwith any suitable liquid fuel control.

Accordingly, a general object of the invention is to pro vide a fuelcontrol adapted to meter suflicient gaseous fuel to the engine tosatisfy the total energy requirements of the engine under all operatingconditions.

Another object of the invention is to provide such a control whichprovides for adjusting the various components thereof in order tocompensate for different gas density and/or B.t.u. values characteristicof fuels from difierent sources throughout various parts of the country.

A further object of the invention is to provide such a control whichmodulates the scheduling hydraulic signal received from the liquid fuelcontrol in accordance with gaseous fuel inlet pressure variations.

Other objects and advantages of the invention will become more apparentwhen reference is made to the following specification and theaccompanying drawings, wherein:

FIGURE 1 is a block diagram layout of a gas turbine engine having a fuelcontrol system embodying the invention;

FIGURE 2 is a schematic illustration of a fuel control embodying theinvention;

FIGURE 3 is a fragmentary view illustrating a modification of a portionof the structure shown by FIG- URE 2;

FIGURE 4 is a graphical representation of a characteristic of theinvention.

3,327,756 Patented June 27, 1967 Over-all engine installation Referringnow to the drawings in greater detail, FIG- URE 1 illustrates astationary industrial gas turbine engine 10 operating any suitabledevice, indicated as load 12. The engine 10 may be supplied liquid fuelby any suitable liquid fuel control 14, the details of construction ofwhich need not be shown, via a conduit 16', the fuel control beingsupplied with liquid fuel from a fuel source 18 via a conduit 20. Inaccordance with the objects of the invention, a gaseous fuel control 22is included in the system.

A conduit 24 communicates between any suitable liquid fuel control 14and the gaseous fuel control 22, and liquid fuel may be diverted fromthe engine 10 to the gaseous fuel control 22 by closing off a valve 26-in the line 16 and opening a valve 28 in the line 24. As will beexplained later, this liquid fuel provides a hydraulic signal to theportion of the gaseous fuel control 22 through which it flows, the fuelultimately flowing through a heat exchanger 29 connected in a conduit 30leading back to the liquid fuel source 18. The signal received by thegaseous fuel control 22 controls, in a manner to be described, the flowof natural gas through the gaseous fuel control 22.

The natural gas is supplied to the control 22 from a gaseous fuel source32 via a conduit 34. A pressure regurlator 36 may be included in theconduit 34. The gaseous fuel control 22, being influenced by thehydraulic signal, will supply natural gas to the engine 10 via a conduit38 in quantities such that the total energy content thereof is the sameas would be the case if liquid fuel were being supplied directly to theengine 10.

Gaseous fuel control The gaseous fuel control 22 includes a housing 40which may consist of several matching parts. In any event, the housing40 includes an inlet 42 and an outlet 44 for the natural gas and aninlet 46 and an outlet 48 for liquid fuel. A main fuel passage 50,including a venturi 52, communicates between the inlet 42 and the outlet44. A rod-like pointed member 54 may extend through a wall 56 of thehousing 40 into the throat of the venturi 52, the member 54 beingmanually adjustable along the axis of the venturi 52 by means of anadjustment screw 58, for a purpose to be explained. A scale 60,indicative of a range of B.t.u. values, is attached to or formed on thehousing 40 adjacent the screw 58, and a marker 62 extends from themember 54 so that it may be aligned with any of the B.t.u. marks 64 onthe scale 60. As the screw 58 is turned the rod 54 moves axially due tothe pin 65 being located in the groove 67 formed in the rod 54; themarker 62, merely moves with the rod 54.

A diaphragm assembly 66 provides a movable wall between chambers 68 and70 formed in the housing 40. A throttling valve 72 secured to thediaphragm assembly 66 includes a circular shoulder 74 which is slidablymounted in a cylindrical passage 76 communicating between the main fuelpassage 50 and the chamber 6-8. A throttling valve head 78 is formed ona stem 80' extending from the shoulder 74 across the passage 50 and isslidably mounted within a cylindrical opening 82, the lattercommunicating between the inlet 42 and the main fuel passage 50 viaopenings 83' formed in the Wall thereof.

A passage 84 communicates between the gaseous fuel inlet 42 and thechamber 68 adjacent one side of the diaphragm 6'6, and a filter 86 ismounted in another chamber 88 adjacent the passage 84. A passage 90communicates between the gaseous fuel inlet 42 and the chamber 88, whileanother passage 92 communicates between the chamber 88 and the chamber70* adjacent the other side of the diaphragm '66. A spring loaded bypassvalve 94 may be mounted in still another passage 96 which communicatesbetween the filter chamber 88 and the passage 92 leading to the chamber70 in the event that the filter 86 becomes clogged. A spring 98 mountedin the chamber 68 between an abutment 100' and the diaphragm assembly 66urges the diaphragm in a direction which decreases the size of thechamber 7 0.

Locating the throttling valve 78/83 upstream of the venturi 52 makespossible a larger gaseous flow signal across the force balance system130. This is due to the increased velocity through the venturi 52resulting from the pressure drop across the opening 78/83.

An additional plurality of chambers 102, 104, 106-and 108 are formedwithin the housing 40, with a diaphragm assembly 110 forming a movablewall between the chambers 102 and 104 and a diaphragm assembly 112forming a movable wall between the chambers 106 and 108. A rod 114,surrounded by a cylindrical spacer 116, connects the diaphragmassemblies 110 and 112 by virtue of the rod being slidably mountedwithin a bushing 118 pressed into an opening 120 extending between thechambers 104 and 106. The ends of the rod 114 extend beyond the spacer116, through the centers of the assemblies 110 and 112 where they arefastened by means of nuts 122 and 124, respectively. A round pin 126 isslidably mounted within a bushing 128 pressed into an opening 129between the chamber 102 and the chamber 70. The elements 112, 114, 116,110, 126 and 66 make up a force balance system 130, which controls thepositioning of the throttling valve 78 in a manner to be described.

A balancing pin 132 is slidably mounted within a hollow member 134threadedly inserted in a Wall of the housing'40 adjacent the chamber108, the pin 132 being-held against the end 124 of the rod 114 in amanner which will be described lated. Openings 136 in the walls of themember 134 communicate between the axial passage 138 thereof and annulus140 surrounding a portion thereof. A passage 142 communicates betweenthe annulus 140 and the chamber 70. A spring retainer 144 is mountedwithin the chamber 108 on the end of the member 134. A spring 146 ismounted between the retainer 144 and the diaphragm assembly 112. Themember 134 may be turned to reposition the retainer 144. An adjustablestop member 146 is threadedly mounted within the member 134 adjacent theend 148 of the balancing pin 132.

A chamber 150 is formed adjacent the main fuel passage 50, and a passage152 communicates between the chamber 150 and the chamber 108. Anotherpassage 154, including a fixed temperature modulating restriction 156,communicates between passage 152 and the throat of the venturi 52, whilestill another passage 158 communicates between the passage 154 and thechamber 102. The fixed restriction 156 is provided in the passage 154between the junctions thereof with the passages 152 and 158. A valveseat 160 is formed in the chamber 150, and a temperature responsivevalve assembly 162 is mounted across the main fuel passage 50, the valve163 portion thereof being urged toward the valve seat 160 by a spring164 which abuts at its one end against a wall of the housing and atitsother end against a retainer 166 formed on the valve 162. A plurality ofbimetallic disks 168 are mounted around the valve stem 162 and betweenthe spring retainer 166 and an abutment 170 mounted at the inlet to thechamber 150. Openings 172 formed in the abutment 170 communicate betweenthe main fuel passage 50 and the chamber 150 upstream of the valve seat160. In the event the temperature of the gas remains constant,

the assembly 162 would not be required and the fixed re striction 156would be replaced by a plug, blocking off communication between thepassages 152 and 158.

Under certain conditions, a valving assembly 174 is threadedly mountedin the liquid fuel inlet 46. A fixed restriction 176 is threadedlymounted in the axial passage 178 formed through the assembly 174. Acheck valve 180 having an opening 182 therethrough different in diameterfrom the opening through the fixed restriction 176 is included in theaxial passage 178 adjacent the fixed restriction 176 and urged away fromthe fixed restriction 176 and toward a fixed hollow abutment 184 by aspring 186. A pair of chambers 188 and 190 are formed in the housing 40,and a perforated wall 192 is included between the chambers 188 and 190.A passage 194 communicates between the liquid fuel inlet 46 and thechamber 188, while a passage 196 communicates between the chamber 190and the liquid fuel outlet 48. A valve seat 198 is formed in the chamber188. A valve member 200 is slidably mounted in a central opening 202 inthe wall 192, the valve member 200 including a contoured center portion204 which serves as the valve and cooperates with the valve seat 198.The valve member 200 is urged into the chamber 190 by a spring 206mounted in the chamber 188 so as to urge the larger diameter portion ofthe contour 204 toward the valve seat 198. A passage 208 including adampening restriction 210 communicates between chamber 188 and thechamber 106. A passage 212 including a second dampening restriction 214communicates between the chamber 190 and the chamber 104, while stillanother passage 216, including a. calibrated fixed restriction 218,communicates between the passages 212 and 208.

A member 219 is slidably mounted within a manually adjustable member220, the latter being threadedly mounted in a wall of the housing 40adjacent the chamber 19.0. A scale 221 is mounted on the housing 40 anda marker 222 extends thereto from the screw 220, for a purpose to bedescribed. As the screw 220 is turned, the pointer 222 moves along thescale 221 by virtue of a pin 215 which is secured to the screw 220 andmovable in the groove 217 formed in the member 219. A stern 223extends-from the adjustable member 220 into the chamber 190, and a lever224 is pivotally mounted on the stem 223. The lever 224, which includesa tapered portion 225, is urged toward the end of the valve member200'by a pin 226 which is slidably mounted within a flanged guide 228held against a wall 230 by a spring 240, the wall 230 being locatedbetween the chamber '190 and an adjacent chamber 232. Openings 234,formed through the guide 228, communicate between the two chambers 190and 232. A diaphragm assembly 236 forms a movable wall between thechamber 232 and an adjacent chamber 238, the assembly 236 being urgedtoward the pin 226 by a spring 239 in the chamber 238, against the forceof the spring 240 in the chamber 232. Due to the forces of the springs239 and 206, and the pressure within the chamber 238, it may be notedthat the valve member 200, the lever 224, the pin 226, and the diaphragmassembly 236 will be maintained in continual series contact. Theretainer 241 for the spring 239 may be manually adjusted by means of ascrew 242. A passage 243 supplies natural gas from the main fuel passage50, just upstream of the venturi 52, and the chamber 238.

As illustrated in FIGURE 3 and as one skilled in the art would normallyconsider to be the better practice, the passage 243 leading to thechamber 238 would preferably communicate with the throat of the venturi52, rather than with the passage 50 at a point upstream-of the venturi52, in which case the following approximate air or gas flow equation,which, over the pressure ratios involved in the invention, is accurateto 99 /2 would apply:

wherein W is the flow of the natural gas, A is the area of the venturi,P is the pressure of the gas upstream of the venturi, P is the pressureof the gas at the throat of the venturi and T is the temperature of thegas. As previously mentioned, if temperature were to remain constant,the temperature adjustment assembly 162 and the fixed restriction 156would not be required.

While the location of the inlet to the passage 243 at the venturi meetsthe requirement of the above equation, for some unexplainable reasontests have indicated that better operation is obtained when the inlet tothe passage 243 is located upstream of the venturi; hence, the preferredembodiment is as illustrated in FIGURE 2, which in effect, changes theabove equation to Operation As mentioned above, when it is desirable tooperate a turbine engine with gaseous fuel, rather than with liquidfuel, liquid fuel enters the inlet 46 (FIGURE 2) of the gaseous fuelcontrol 22 from the conduit 24, having been diverted from the conduit 16by the manipulation of the valves 26 and 28. The check valve assembly 174 may be an accessory item which is threadedly inserted in the inlet 46only when a particular engine 10 normally receives liquid fuel from theconduit 16 through a so-called duplex nozzle (not shown). A duplexnozzle serves to back-pressure the liquid being received from the liquidfuel control 14 to produce two different pressure v. fuel flow ratios,such as illustrated by the solid line curve, FIGURE 4. Since the liquidfuel is being diverted from the conduit 16 to the conduit 24, it isessential that it be back-pressured in a manner similar to theback-pressuring effect of the duplex nozzle. The valve assembly 174fulfills this requirement in that, once the fuel flow is such that somepredetermined pressure drop exists across the valve 180, comparable topoint A, FIGURE 4, the valve will be lifted oif the abutment seat 184and yet held away from the fixed restriction 176 by the spring 186. Fuelflow will now be controlled by the larger diameter opening, say, .060",through the fixed restriction 176, rather than by the considerablysmaller opening 182, say, .020", through the valve 180. The resultingpressure v. fuel flow relationship, while not exactly the same as thatof a duplex nozzle, will be, for all practical purposes, substantiallythe same, as indicated by the dash line curve of FIGURE 4.

Liquid fuel which thus enters the inlet 46 flows through the passage194, into the chamber 188, past the valve 200 into the chamber 190, andthrough the passage 196 to the outlet 48, from which it then returns tothe liquid fuel source 18 (FIGURE 1) by way of the conduit 30. Thepressure of the liquid in the chamber 188 is transmitted to the chamber106 by way of the passage 208 and the dampening restriction 210, thelatter being provided in order to restrict the effect of any pulsatingflow on the diaphragm 112. The pressure of the liquid fuel in thechamber 190, being indicative of the pressure drop through the valve204/198, is transmitted to the chamber 104 via the passage 212 and thedampening restriction 214, the latter serving the same purpose as thatof the restriction 210.

Ignoring, for the moment, the effect of the gas density adjustment screw220, the exact position of the contoured valve 200 is influenced in partby the gaseous fuel pressure in the chamber 238, which not only reflectseither venturi 52 throat pressure P or upstream pressure P as previouslyexplained, but which is modulated by compressor discharge pressure dueto back-pressure from the compressor through the conduit 38, the venturi52 and the passage 243. The pressure in the chamber 238 thus moves thevalve 200 through the diaphragm 236, the pin 226 and the lever 224,thereby modifying the fuel signal to the chambers 104 and 106 inresponse to changes in com- 6 pressor discharge pressure. The adjustmentscrew 242 is provided in order to attain a desired acceleration curveshape by adjusting the position of the valve 200 after assembly, therebyeliminating any errors due to the stackup of tolerances of elements 200,224, 226 and 236.

The fixed restriction 218 is provided as a calibration aid, in order tomove the acceleration curve up or down, without appreciably affectingits shape, for a proper fuel flow vs. speed relationship. It is thecombined pressure drop across the valve 204/198 and the restriction 218which results in a difference in pressures in the chambers 104 and 106.This pressure differential is proportional to the fuel flow output ofthe liquid fuel control device 14 and serves as the hydraulic signalwhich, in effect, indicates to the force balance system 130 just howmuch fuel should be going through the gas control. The resultant forceon the diaphragms 112 and is toward the right in FIGURE 2, since agreater pressure exists in the chamber 106 than in the chamber 104.

The natural gas which enters the inlet 42 from the conduit 34 flows pastthe valve 78, through the openings 83 and thence through the main fuelpassage 50, the venturi 52 and the outlet 44, to the engine 10 via theconduit 38. As already explained, the resultant force of the systemincluding diaphragms 110 and 112 due to the pressure of the liquid fuel,which is indicative of the gaseous fuel flow desired, is eventuallybalanced by an opposing resultant force due to the above describedgaseous flow. The opposing force is the result of gaseous fuel from themain passage 50 flowing through the openings 172, past the valve /163,into the chamber 108 via the passage 152 and into the chamber 102 viathe passage 158 and the fixed temperature modulating restriction 156.The force is affected by the pressure drop occurring at the venturi 52,the latter effect being transmitted to the passage 158 via the passage154. Hence, the total drop across the venturi 52 and the restriction 156is proportional to the actual gaseous fuel flow and results in apressure differential in the chambers 102 and 108, tending to move thediaphragms 110 and 112 to the left in FIGURE 2 in opposition to therightward movement due to the liquid fuel pressure force as previouslymentioned.

It should noW be apparent that any time that a change in liquid fuelflow is produced by the various parameters employed in the operation ofthe liquid fuel control 14 and that the change is reflected in thechambers 104 and 106, causing the valve 78 to be moved either toward oraway from the cylindrical opening 82. This varies the openings 83 andresults in either less or more gaseous fuel flowing therethrough. As thegaseous fuel flow in the main passage 50 is thus decreased or increased,the pressure change will be reflected in the chambers 102 and 108, andthe valve 78 will once again move until the resultant liquid fuel forceto the right exactly balances the resultant gaseous fuel force to theleft.

In order that the force balance system 130 reflect only pressuredifferences proportional to the liquid and gaseous fuel flows, thepressure on the front face of the valve 78 and in the chamber 68 againstthe diaphragm 66 is washed-out by the pressure within the chamber 70against the other side of the diaphragm 66, plus the pressure againstthe end 148 of the balancing pin 132. In other Words, the totaleffective pressure acting upon the left side of the diaphragm 66 isexactly equal to the total effective pressure acting on the right sideof the diaphragm 66. The adjustable stop member 146 is provided in orderto protect the engine 10 from pressure surges. Also, the position of thespring retainer 144 may be adjusted by turning the member 134.

The filter 86 in the chamber 88 is included to insure that only cleangaseous fluid be supplied to the chambers 70 and 138. Thus, any gaswhich escapes past the pins 126 and 132 as a result of their beingslidably fitted within the bushings 128 and 134, respectively, will notcontaminate the sliding surfaces.

In those applications wherein the temperature of the gas is subject tofluctuations, the temperature compensating valve assembly 162 may beemployed. As temperature increases, there will be a correspondingdecrease in density of the gaseous fuel. This decrease in density willbe compensated for by virtue of the increased temperature causing thebimetallic disks 168 to expand and thereby decreasing the pressure dropacross the opening 163/160. The leftward resultant force (FIGURE 2) ofthe system 130 willbe decreased, producing a larger opening 78/83 and acorresponding increased flow of the less dense gaseous fuel. Of course,a decrease in temperature will have the opposite effect.

-When a different supply of natural gas is used, it may very well bethat its heat content will be difierent. If a B.t.u. adjustmentmechanism 58/54 is employed, the change in heat content may becompensated for by resetting the position of the tapered rod 54. This isaccomplished simply by turning the screw 58 until the new B.t.u. value,corresponding to the reading of a calorimeter, for example, is indicatedby the marker 62 on the scale 60. If it becomes necessary to adjust froma setting of 800 B.t.u. (lean) to 1600 B.t.u. (rich), for example, it isapparent that the pointed end of the rod 54 would extend further intothe venturi 53, thereby decreasing the size of the opening therethrough.This would result in a greater fuel velocity through the venturi, andthe consequent reduced pressure at the venturi 52 would cause acorresponding decrease in pressure in the chamber 102. This, of course,will move the throttling valve 78 to the left and reduce the fuel flowthrough the opening 83. Thus, fuel flow to the engine 10 will besufficient to meet the total energy requirements thereof.

Should it be determined that the density of the gaseous fuel haschanged, as due to a different fuel supply, for example, the screw 220may be turned to a new density reading on the scale 221. This will movethe valve 200 due to its engagement with the sloped portion of the lever224, resulting in a change in the pressure differential in the chambers104 and 106 and causing a repositioning of the valve 78 accordingly.Thus, the gaseous fuel flow to the engine 10 will once again be changedto supply the engine with its total energy requirements, lower densitiesrequiring greater fuel flow and higher densities requiring less fuelflow for a particular energy requirement. Where no provision for densityadjustments is desired, the lever 224 need not include the shapedportion 225 and, in fact, could be eliminated if the chambers 232 and238 .were arranged so that the sliding pin 226 were aligned with and indirect contact with the end of the valve 200.

It would be possible to produce a B.t.u. adjustment with the adjustingscrew 220 and to compensate for density changes'by adjusting the screw58; in other words, with a recalibration, the density and B.t.u.adjustment functions could be reversed.

It should be noted that the gaseous portion of the fuel control 22 is alow pressure system and need only be supplied with natural gas at apressure greater than the compressor discharge line pressure in conduit38. In a typical application, the compressor discharge pressure will beapproximately five times atmospheric pressure; under such circumstances,the minimum pressure at which the gaseous control will function is onthe order of 75 p.s.i.

It should be apparent from the above discussion that the invention hasprovided a means whereby stationary and turbine engines may be operatedwith either liquid fuel or gaseous fuel by simply switching from one tothe other through a suitable valving arrangement. As described, when itis desirable to use gaseous fuel, the liquid fuel control will continueto operate in order to provide a hydraulic signal to the gaseouscontrol, but will merely recirculate liquid fuel back to the reservoir.

It should be further apparent that the basic gaseous control unit may bemade more versatile by the inclusion of accessory features which mayeither besubsequently added to the basic control or which may beprovided as original equipment, in order to correct gaseous fuel flowfor changes in temperature, density and/ or heat content (B.t.u.)variations.

It should be apparent from the above discussion that the inventionprovides a variety of improvements over prior art gaseous fuel controls.

While but two embodiments, along with various possible adjustmentfeatures, of the basic gaseous control have been shown and described forpurposes of illustration, it is apparent that other modifications of theinvention may be possible within the scope of the appended claims.

What we claim as our invention is:

1. For use with a gas turbine engine having a liquid fuel controldevice, a gaseous fuel control device comprising a force balance systemincluding two pairs of chambers, each pair of chambers having a pressureresponsive device forminga movable Wall therebetween, a stern connectingthe two pressure responsive devices, and a throttling valve operativelyconnected to said stem for movement therewith; a liquid fuel inlet, aliquid fuel outlet, a passage communicating therebetween, a compensatingvalve in said passage and a pair of passages communicated between one ofeach of said pairs of chambers and said liquid fuel passage at pointsupstream and downstream of said compensating valve; a gaseous fuelinlet, a gaseous fuel outlet, a main fuel flow passage therebetween, aventuri formed in said main fuel passage and a pair of conduitscommunicating between the other of each of said pairs of chambers andsaid main gaseous fuel passage at the throat of said venturi and at apoint upstream thereof, and means associatedwith said venturi forchanging the gaseous fuel flow therethrough to compensate for changes inheat content of the gaseous fuel.

2. For use with a gas turbine engine having a liquidfuel control device,a gaseous fuel control device comprising a force balance systemincluding two pairs of chambers, each pair of chambers having a pressureresponsive device forming a movable wall therebetween, a stem connectingthe two pressure responsive devices, and a throttling valve operativelyconnected to said stern for movement therewith; a liquid fuel inlet, aliquid fuel outlet, a passage communicating therebetween, a compensatingvalve in said passage and a pair of passages communicated between one ofeach of said pairs of chambers and said liquid fuel passage at pointsupstream and downstream of said compensating valve; a gaseous fuelinlet, a gaseous fuel outlet, a main fuel flow passage therebetween, aventuri formed in said main fuel passage and a pair of conduitscommunicating between the other of each of said pairs of chambers andsaid main gaseous fuel passage at the throat of said venturi and at apoint upstream thereof, and means associated with said venturi forchanging the gaseous fuel flow therethrough to compensate for changes indensity of the gaseous fuel.

3. The device described in claim 2, and including, additionally, a meansfor modulating the liquid fuel signal past said compensating valve inaccordance with changes in the inlet pressure of the gaseous fuel.

4. The device described in claim 3, wherein said modulating meansincludes a pair of chambers, a pressure .responsive device forming amovable Wall between said chambers, connecting means between saidpressure responsive device and said compensating valve, and a passagecommunicating between one of said pair of chambers and said main gaseousfuel flow passage at a point upstream of said venturi.

5. In'combination with a stationary gas turbine engine having a liquidfuel control device and capable of operation on gaseous fuel, a gaseousfuel control device, said device comprising a force balance system,means for using the scheduling signal from the liquid fuelcontrol deviceto operate said force balance system of said gaseous :fuelcontroldevice, means for switching from the 5? liquid fuel controldevice to said gaseous fuel control device, or vice versa, to provide atrandom an amount of gaseous fuel equivalent to the amount of liquid fuelto said turbine engine, means for adjusting the gaseous fuel flowthrough said gaseous fuel control device according 5 to the specificB.t.u. content thereof, means for adjusting the fuel flow through saidgaseous fuel control device to compensate for changes in gas density dueto changes in temperature of said gaseous fuel and means for modulatingthe gaseous fuel flow to compensate for a change in the gas density.

References (Jited UNITED STATES PATENTS 2,931,429 4/1960 Brown 153113,241,597 3/1966 Juzi 15811 X FREDERICK L. MATTESON, JR., PrimaryExaminer. E. G. FAVORS, Assistant Examiner.

1. FOR USE WITH A GAS TURBINE ENGINE HAVING A LIQUID FUEL CONTROLDEVICE, A GASEOUS FUEL CONTROL DEVICE COMPRISING A FORCE BALANCE SYSTEMINCLUDING TWO PAIRS OF CHAMBERS, EACH PAIR OF CHAMBERS HAVING A PRESSURERESPONSIVE DEVICE FORMING A MOVABLE WALL THEREBETWEEN, A STEM CONNECTINGTHE TWO PRESSURE RESPONSIVE DEVICES, AND A THROTTLING VALVE OPERATIVELYCONNECTED TO SAID STEM FOR MOVEMENT THEREWITH; A LIQUID FUEL INLET, ALIQUID FUEL OUTLET, A PASSAGE COMMUNICATING THEREBETWEEN, A COMPENSATINGVALVE IN SAID PASSAGE AND A PAIR OF PASSAGES COMMUNICATED BETWEEN ONE OFEACH OF SAID PAIRS OF CHAMBERS AND SAID LIQUID FUEL PASSAGE AT POINTSUPSTREAM AND DOWNSTREAM OF SAID COMPENSATING VALVE; A GASEOUS FUELINLET, A GASEOUS FUEL OUTLET, A MAIN FUEL FLOW PASSAGE THEREBETWEEN, AVENTURI FORMED IN SAID MAIN FUEL PASSAGE AND A PAIR OF CONDUITSCOMMUNICATING BETWEEN THE OTHER OF EACH